a. Field of the Invention
This invention relates to controlling adhesive bond strength of orthodontic brackets. In particular, this invention pertains to the coating of the base of a ceramic orthodontic bracket that will be chemically bonded to teeth in order to control the adhesive bond strength between the bracket and a tooth. The invention is particularly applicable to bonding where silane coupling is used.
b. Description of the Prior Art
Ceramic brackets for adhesive bonding to teeth are a recent development in the field of orthodontics. Such materials are of high structural strength so as to provide a superior product that will last over the typical two to three years length of treatment. In addition, these materials have optical properties that render them translucent or transparent. This provides a cosmetic advantage of being inconspicuous in the mouth.
Methods of adhering ceramic brackets to teeth can be broken down into mechanical bonding and chemical bonding. Mechanical bonding includes: (1) products furnished with channels and holes in the adhesive surface, (2) products furnished with multiple projections on the adhesive surface, an (3) products with fine particles of ceramic or glass sprinkled over and adhered to the adhesive surface. Chemical bonding includes: (1) products with silane couplings painted on the adhesive surface, (2) products with the base surfaces covered with a silica glass that bonds chemically with the adhesive, and (3) products where the base elements are formed of plastic and this plastic bonds chemically with the dental adhesive.
Chemical bonding using silane couplings are very strongly adhered to tooth surfaces. This high adhesive bond strength prevents the bracket from peeling away from the tooth during a normal treatment period, however, it can also make bracket removal difficult.
A method for controlling the bond strength between the inorganic bracket and the organic adhesive using silane coupling is disclosed in U.S. Pat. No. 4,948,366. That approach is to apply a mixture of organofunctional silane coupling agents, one which is reactive, the other non-reactive with the organic resin of the adhesive. When the organic adhesive is applied, it reacts with the silane coupling agent in proportion to the amount of reactive organofunctional silane coupling agent present. According to the patent, by varying the proportions of reactive and non-reactive silane coupling components of the mixture, the bond strength can be varied. Another method proposed to debond ceramic brackets involves the application of heat to the bracket by means of a short time contact. The application of heat must however be controlled so that it is sufficient to soften the adhesive but not to cause damage to the tooth. While this technique and others have met with some degree of success, none provide an optimal solution to the present disadvantages of debonding.
During debonding, force is applied to the base or the wings of a bracket to overcome the bond strength of the adhesive. This applied force does not, however, create a constant stress across the entire bonded area. Debonding techniques create differential stresses across the tooth enamel, in the adhesive, and in the bracket. The type and location of the stress created varies depending upon the debonding method used. The adhesive-bracket interface can be broken, leaving adhesive on the tooth surface that must be removed with a rotary grinder. More serious problems arise when the adhesive-enamel interface is broken because the tooth enamel can be damaged.